The present invention is with respect to a process for evening out the temperatures in the upper and lower parts of a charge in the preheating part of a ceramic tunnel kiln by causing thorough mixing of the kiln gases. Furthermore, the invention is with respect to an apparatus which may be used for undertaking the process.
If no special steps are taken with respect to heat transfer in the preheating zone so that such transfer is only caused by the draft in the kiln, there will be, as has been seen from experience, a difference of commonly 300.degree. C. between the temperatures in the top and lower kiln charge. It is only at the start of the firing or burning zone that there is a direct firing effect producing an equal temperature over the full kiln cross-section, this being responsible, in turn, for the temperature at the lower part of the charge and at the inlet end of the firing zone being very much more quickly increased than the temperature in the top part of the charge. The outcome may commonly be damage such as thermal cracking or the like of the goods in the lower part of the charge which are to be fired. On the other hand, the goods forming the top part of the charge, do not, as a rule, undergo such damage.
Although attempts have been made at taking care of this shortcoming by aspirating the kiln gases from the top of the preheating zone and then blowing them back into the lower part of the kiln, such circulation, while causing a certain mixing and temperature-evening effect, only makes for a temperature level which is at an average temperature value between the lowest and highest temperatures of the kiln gases mixed together. For this reason, as a rule, the temperatures likely to be produced are not high enough for stopping damage to the goods to be fired at the inlet end of the firing zone because of heating up therein taking place over-quickly. For this reason, the throughput rate has to be lowered if no waste is to be produced. The outcome of this is, however, that the profit produced on working the kiln will be cut back.